A conventional navigation apparatus to navigate has a storage medium such as a hard disk drive (HDD), or DVD-ROM to store road map data. The HDD has a data storage capacity of several tens to several hundreds GB (Giga Byte); the DVD-ROM, several to several tens GB.
The HDD or DVD-ROM has such a large storage capacity enough to store road map data having a larger data volume, thereby being mainstream in use for storing road map data.
In contrast, a navigation apparatuses using a CD-ROM/RAM as a storage medium for storing road map data is still present. In addition, a navigation apparatus using a card-type or chip-type flash memory (called a memory card) for storing road map data has been developed. This is because a memory card is easy to rewrite data while the data capacity has been recently increased.
Note that the storage capacity of a memory card is still smaller (several GB) than that of the HDD or DVD-ROM (several hundreds GB) although having been increased rapidly.
Thus, in many cases, the road map data generated for a HDD or DVD-ROM cannot be directly stored or transferred in a memory card or CD-ROM/RAM without modification. If the road map data prepared for a HDD or DVD-ROM (called large road map data) is used for a memory card or CD-ROM/RAM, the following methods can be used, for example.
(1) Deleting data which is deletable in the large road map data; or
(2) Transforming a data format of the large road map data to a different one which can reduce the data volume.
For example, in the method (1), partial road map data corresponding to unnecessary areas is deleted while data corresponding to the necessary area remains; further, data for audio guidance is deleted.
While any Patent document explicitly disclosing the above methods (1) (2) is not retrieved, Patent document 1 describes an idea to delete unnecessary data among the road map data stored in a HDD.    Patent document 1: JP-2001-165669 A (corresponding to U.S. Pat. No. 6,556,920)
Adopting the method (1) may lower the quality of the road map data. The satisfaction of a user therefore seems to be limited or lowered.
In addition, adopting the method (2) may require to start from the definition or development of the new format which can reduce data volume in some cases. In the event of using the existing format, the transformation of the format might be rather difficult, requiring additional many man days. For example, the road map data after the transformation should require inspection to check for abnormalities or errors.